This invention relates to an optical information record medium such as an magneto-optical recording disk, write once type optical disk, compact disks, etc., and more particularly to an improved polycarbonate resin used as a substrate material for such record medium.
The optical record system provides such characteristics, or advantages, as handiness that enables non-contact recording/reproduction, high durability against dirt or scratches, and a storage capacity several tens or hundreds times greater than a magnetic record system, for example, that they are not only utilized as so-called compact disks wherein audio signals signals are digitally recorded or video, but also are expected to be used in filing code information or image information with great storage capacity.
Among the optical information record medium, there is a wide variety of write once type optical disks, or magneto-optical disks, other than the aforementioned compact disks. However, they all have an optical information record layer formed on a transparent substrate. Furthermore, the substrate material must meet certain requirements and characteristics. For instance, the following features of the substrate material are indispensable: (1) a material having thermal resistance at the time of melt molding and can be molded with ease; (2) a material that is not deformed or changed in quality after disk molding, (3) a material having supreme mechanical characteristics, (4) a material having a small double refraction due to, for example, a molecular orientation, a matter of which is important during recording/reproduction. The following materials meet these demands: poly methyl methacrylate resins, polycarbonate resins, polymethyl pentene resins, or polystyrene resins, and so on.
Among them, polycarbonate resin is superior with respect to the aforementioned points (1) to (3), and is the most hopeful substrate material of the optical information medium. For example, in Japanese Patent Publication without examination Nos. 60-155424, 61-4726, 61-16962, 61-19656, 61-55116 and 61-55117, there are disclosed optical disks with polycarbonate resins.
However, the polycarbonate resin suffer the disadvantage of having a large double refraction at the time of disk molding because of its molecular structure. Particularly, it is indispensable for an magneto-optical disk, for example, to minimize the double refraction caused in the transparent substrate, thus the occurence of the double refraction is a serious problem for it.
Therefore, in order to use a polycarbonate resin as the transparent substrate of this kind, one must reduce the double refraction that occurs at the time of molding.